bug#41321: 27.0.91; Emacs aborts due to invalid pseudovector objects

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Fri, 29 May 2020 10:34:20 +0000
> Cc: Paul Eggert <eggert@cs.ucla.edu>, Stefan Monnier 
> <monnier@iro.umontreal.ca>, 41321@debbugs.gnu.org
> 
> > . There's nothing wrong with GDB's xtype command: it fails when a Lisp
> >   object encodes a pointer to invalid memory:
> 
> (gdb) p last_marked[497]
> $100 = XIL(0x439c370)
> (gdb) xtype
> Lisp_Vectorlike
> Cannot access memory at address 0x1ffac2a8
> 
> Again, that can't be right. $100 is a Lisp_Symbol, not a vectorlike,
> and it's not at address 0x1ffac2a8.
> 
> So my suspicion remains that this is a gdb bug, and it appears to be a
> reproducible one!

There's no bug: the $size variable was not updated inside pvectype
because the 'set' command tried to access invalid memory.  So the rest
is using the stale value of $size.  Puff! no miracle and no bug.

You just don't need to assign too much importance to the address the
error message displays, it might not be the problematic address.